This invention relates to ECCM systems and more particularly to selectable means for countering the effects of ECM which deceives the AGC circuits of the radar receiver.
In radar systems and particularly tracking radars using conical scan of the received or echo beam, the radar processor measures the angular difference between the radar target position and the beam position. When a beam pointing error occurs, the scanning process applies a sinusoidal amplitude modulation to the target echo energy. The radar signal processor measures the index of modulation at the conical scan rate and uses this information to drive the tracking loop servo to keep the radar beam pointed at the target. Such conical scanning tracking radars are more fully described in the text Introduction To Radar Systems, by Merrill I. Skolnik., published by the McGraw-Hill Book Company in 1962, Section 5.3. Radars employing conical scan, as well as any other radar system which extracts information contained within the amplitude modulation of a carrier, can be degraded or defeated by providing a supplementary premodulation which is equivalent to periodic enhancement of a signal at an arbitrary level. The requirements for effective ECM activity are that the enhancement be large and that the enhancement be accomplished at a duty ratio equal to the reciprocal of the victim signal processor dynamic range. Some prior ECCM radar systems utilize a plurality of radar stations coupled to a computer to compute the intersections of a jamming target with all stations to position the target. Other ECCM radar systems utilize circuitry to differentiate signal voltages in at least two receiver channels to produce an output only upon coincidence of signals and when the sense of the signals are in the same direction to distinguish true target signals. In general the usual approach of combating ECCM is to avoid saturating or overloading the radar receiver with large interfering signals and this is often accomplished with proper shielding and extensive filtering to increase signal-to-noise (SIN) ratio.